The invention further relates to a grating image and a security document, such as bank notes, ID cards, or the like, with such a grating image.
Holograms, holographic grating images and further optically variable diffraction structures are used as security elements against forgeries in credit cards, bank notes, product packaging and the like. In general the manufacturing of these diffraction structures begins with exposing a light-sensitive layer to overlapping, coherent light beams.
Real holograms are formed by illuminating an object with coherent laser light, the laser light disturbed by the object being overlapped with an undisturbed reference beam in the light-sensitive layer.
Holographic diffraction gratings are the result, when the light beams overlapping in the light-sensitive layer consist of spatially expanded, uniform, coherent wave fields. When these are allowed to act upon the light-sensitive layer, e.g. a photographic film or a photoresist layer, the result is a holographic diffraction grating, which e.g. in a photographic film is preserved as bright and dark lines or in a photoresist layer as peaks and valleys. Because the light beams in this case are not disturbed by an object, the only result is an optically variable colour effect, but not the display of an image.
Holographic grating images can be produced out of holographic diffraction gratings, when not the entire surface of the light-sensitive material is covered with a uniform holographic diffraction grating, but masks are used, so that only parts of the recording area are covered with a uniform grating pattern, while other parts of the recording area may be covered with other grating patterns with the help of other masks. A holographic grating image thus is composed of several grating fields with different diffraction grating patterns. With a grating image formed out of the sum of the grating fields most different image motifs can be depicted.
The diffraction gratings of a holographic grating image, usually, are ruled gratings with a multitude of grating lines located side by side. All diffraction gratings of each grating field or image field of the grating image are characterized by the grating constant, the azimuth angle and the contour or the outline. The grating constant here corresponds to the distance between the grating lines and the azimuth angle describes the inclination of the grating lines with respect to a reference direction. The grating constant and the azimuth angle are determined by the wavelength and the incidence direction of the wave fields used for exposing. The outlines of the image fields are produced with the aid of masks.
The grating constants of the grating patterns in the individual image fields are essential for the colours in the grating image, while the azimuth angles of the grating patterns are responsible for the visibility of these image fields from certain viewing angles. On the basis of this technique thus optically variable images, e.g. moving images or also plastically appearing images can be produced.
In general terms there can be stated, that a real hologram is an overlapping of holographic diffraction gratings, whereas in a holographic grating image several holographic diffraction gratings are disposed side by side. In general, real holograms appear photographically true-to-life compared to grating images. Grating images, however, can be designed graphically. In addition, grating images are more light intensive than real holograms, since the undisturbed diffraction gratings located side by side shine more intensive than the overlapping disturbed diffraction gratings.
The diffraction gratings located side by side can be holographically produced in different ways. One possibility is to divide the grating image into large-surface image fields and develop covering masks for these, which permit only one image field to be exposed at a time to form a uniform holographic diffraction grating. Or the entire grating image is divided into a multitude of small, nearly dot-shaped areas, these dot areas having a diameter of 10 to 200 micrometer. In the dot areas then with the aid of a dot matrix machine holographic diffraction gratings can be formed.
In particular with finely structured grating fields with different grating data the mask method is cumbersome in handling. Since the masks have to be brought into very close contact with the light-sensitive layer during the exposure process and positioned very precisely, which requires manual skill and dexterity.
But the manufacturing of the grating images divided into dot areas causes problems as well. The grating images divided into dot areas in fact can be produced automatically by machine and without requiring manual skill, but then the intensity of the reflected light is reduced by the spacings between the dot areas. Furthermore, the colours of the reflected light are distorted, since the gratings are composed in a small-surface fashion and are not uniform over a large surface. Further disadvantages are the perceptibility of the divided dots when viewed under a magnifying glass as well as the low degree of security, because dot matrix machines are easily available.
Furthermore, it is known to produce the dot areas of a quasi-screened grating image by means of an electron beam. The aforementioned disadvantages in connection with the grating image being divided into dots, however, apply in the same way also to this manufacturing variant.